PROJECT SUMMARY Vitamin A or retinol is amongst the most well characterized food-derived nutrients with diverse immune- modulatory roles. Deficiency in dietary vitamin A has not only been associated with immune dysfunctions in the gut, but also with several systemic immune disorders. To maintain sufficient levels of vitamin A, the body relies on the uptake of retinol from the intestinal lumen by intestinal epithelial cells or IECs. After uptake by IECs, retinol can either be processed for storage or be further metabolized into retinoic acid (RA). Even though IECs are at the center of Vitamin A metabolism and play a dominant role in controlling the fate of dietary vitamin A, our knowledge of how IEC intrinsic vitamin A metabolic machinery is regulated is extremely superficial. By comparing germ-free (GF) and conventional (CV) mice we demonstrate that gut bacteria play a critical role in modulating retinoic acid (RA) levels and expression of vitamin A metabolic machinery in the intestinal epithelium. Specifically, we find bacteria differentially regulate expression of retinol dehydrogenase 7 (rdh7), a key gene involved in conversion of retinol into RA in the intestinal epithelium. By employing genetic mouse models harboring a deletion in rdh7 in IECs, we establish that IEC intrinsic RA production regulates the levels of Interleukin IL-22, a key cytokine that controls barrier responses to gut bacteria. Our data demonstrates for the first time that gut bacteria-dependent RA synthesis is critical for regulating local immune responses. Current proposal seeks to understand the molecular mechanism by which gut bacteria regulate RA synthesis and vitamin A metabolic gene rdh7 in IECs (Aim1), establish the role of rdh7 expression in IECs in regulating RA-signaling in the gut (Aim 2) and determine the role of IEC-intrinsic vitamin A metabolism on host-microbe homeostasis via modulation of IL-22 levels in the gut (Aim 3). To accomplish these goals, we have developed innovative genetic approaches that enable us to conditionally delete or over-express rdh7 specifically in IECs. Further we propose to utilize novel methodologies such as monoassociated and gnotobiotic mouse models to delineate the bacterial cues that regulate RA generation in the intestinal epithelium. Our experimental methodology incorporates robust analytical approaches such as tandem mass-spectrometry (LC-MS/MS) that enables us to accurately determine the flux in vitamin A metabolic pathway in response to discrete bacterial cues. This work will be significant because it will establish mechanistic links between gut bacteria and intestinal epithelium intrinsic vitamin A metabolic pathway. Moreover, it will test the feasibility of manipulating mucosal immunity by bacterial modulation of IEC intrinsic vitamin A metabolism for therapeutic purposes.